Unfortunately you are not allowed to ask multiple questions on enotes so I have had to edit your question to only focus on your principal question concerning the identity of the lesser character in this excellent short story by Guy de Maupassant.

Your question seems to assume that there is only one lesser character apart from the protagonist of the piece, Mathilde Loisel. However, there are two: Monsiue Loisel and Mathilde's childhood friend, Madame Forestier, from whom Mathilde borrows the necklace that becomes the source of all her woes. By far the more interesting lesser character is Madame Forestier, who performs two functions in this story. She is the character who lends Mathilde the supposedly diamond necklace, and then at the end, she both comments on the massive change that poverty has brought to Mathilde and informs her that the diamond necklace was fake:

Madame Forestier, quite overcome, clasped her by the hands. "Oh, my poor Mathilde. But mine was fake. Why, at the most it was worth only five hundred francs!"

Thus in this story the two lesser characters are Monsieur Loisel and Madame Forestier. Both of course have their own particular role in this darkly humorous and ironic short story.